Efficient production of L-asparaginase in Bacillus licheniformis by optimizing expression elements and host / 生物工程学报

L-asparaginase (L-ASN) is widely applied in the treatment of malignant tumor and low-acrylamide food production, however, the low expression level hampers its application. Heterologous expression is an effective strategy to increase the expression level of target enzymes, and Bacillus is generally used as the host for efficient production of enzymes. In this study, the expression level of L-asparaginase in Bacillus was enhanced through optimization of expression element and host. Firstly, five signal peptides (SPSacC, SPAmyL, SPAprE, SPYwbN and SPWapA) were screened, among which SPSacC showed the best performance, reaching an activity of 157.61 U/mL. Subsequently, four strong promoters (P43, PykzA-P43, PUbay and PbacA) from Bacillus were screened, and tandem promoter PykzA-P43 showed the highest yield of L-asparaginase, which was 52.94% higher than that of control strain. Finally, three Bacillus expression hosts (B. licheniformis Δ0F3 and BL10, B. subtilis WB800) were investigated, and the maximum L-asparaginase activity, 438.3 U/mL, was reached by B. licheniformis BL10, which was an 81.83% increase compared with that of the control. This is also the highest level of L-asparaginase in shake flask reported to date. Taken together, this study constructed a B. licheniformis strain BL10/PykzA-P43-SPSacC-ansZ capable of efficiently producing L-asparaginase, which laid the foundation for industrial production of L-asparaginase.

Metabolic engineering of L-cysteine supply modules for enhanced production of bacitracin in Bacillus licheniformis / 生物工程学报

Bacitracin is a broad-spectrum antibiotics mainly produced by Bacillus, and is used as veterinary medicine in the fields of livestock and poultry breeding. Insufficient supply of precursor amino acids might be an important factor that hinders high-level microbial production of bacitracin. We investigated the effect of strengthening L-cysteine supply on bacitracin production by an industrial bacitracin producer, Bacillus licheniformis DW2. Overexpression of cysK encoding L-cysteine synthase led to a 9.17% increase of the bacitracin titer. Moreover, overexpression of cysE encoding L-serine acetyltransferase and cysP encoding thiosulfate/sulfate intracellular transporter increased the bacitracin titers by 7.23% and 8.52%, respectively. Moreover, overexpression of a putative cystine importer TcyP led to a 29.19% increase of intracellular L-cysteine, and bacitracin titer was increased by 7.79%. Subsequently, the strong promoter PbacA was used to replace the promoters of genes cysP, cysE and tcyP in strain DW2::ysK, respectively. The resulted strain CYS4 (DW2::cysK-PbacA-(cysP)-PbacA(cysE)- PbacA(tcyP) produced 910.02 U/mL bacitracin, which was 21.10% higher than that of the original strain DW2 (747.71 U/mL). Together with the experiments in 3 L fermenters, this research demonstrated that enhancing intracellular L-cysteine supply is an effective strategy to increase bacitracin production of B. licheniformis.

Progress and perspective on development of non-model industrial bacteria as chassis cells for biochemical production in the synthetic biology era / 生物工程学报

The development and implement of microbial chassis cells can provide excellent cell factories for diverse industrial applications, which help achieve the goal of environmental protection and sustainable bioeconomy. The synthetic biology strategy of Design-Build-Test-Learn (DBTL) plays a crucial role on rational and/or semi-rational construction or modification of chassis cells to achieve the goals of "Building to Understand" and "Building for Applications". In this review, we briefly comment on the technical development of the DBTL cycle and the research progress of a few model microorganisms. We mainly focuse on non-model bacterial cell factories with potential industrial applications, which possess unique physiological and biochemical characteristics, capabilities of utilizing one-carbon compounds or of producing platform compounds efficiently. We also propose strategies for the efficient and effective construction and application of synthetic microbial cell factories securely in the synthetic biology era, which are to discover and integrate the advantages of model and non-model industrial microorganisms, to develop and deploy intelligent automated equipment for cost-effective high-throughput screening and characterization of chassis cells as well as big-data platforms for storing, retrieving, analyzing, simulating, integrating, and visualizing omics datasets at both molecular and phenotypic levels, so that we can build both high-quality digital cell models and optimized chassis cells to guide the rational design and construction of microbial cell factories for diverse industrial applications.

Enhanced production of bacitracin via energy metabolism engineering in Bacillus licheniformis DW2 / 生物工程学报

Bacitracin is a broad-spectrum cyclic peptide antibiotic, and mainly produced by Bacillus. Energy metabolism plays as a critical role in high-level production of target metabolites. In this study, Bacillus licheniformis DW2, an industrial strain for bacitracin production, was served as the original strain. First, our results confirmed that elimination of cytochrome bd oxidase branch via deleting gene cydB benefited bacitracin synthesis. Bacitracin titer and ATP content were increased by 10.97% and 22.96%, compared with those of original strain, respectively. Then, strengthening cytochrome aa3 oxidase branch via overexpressing gene qoxA was conducive to bacitracin production. Bacitracin titer and ATP content were increased by 18.97% and 34.00%, respectively. In addition, strengthening ADP synthesis supply is also proven as an effective strategy to promote intracellular ATP accumulation, overexpression of adenosine kinase DcK and adenylate kinase AdK could all improve bacitracin titers, among which, dck overexpression strain showed the better performance, and bacitracin titer was increased by 16.78%. Based on the above individual methods, a method of combining the deletion of gene cydB and overexpression of genes qoxA, dck were used to enhance ATP content of cells to 39.54 nmol/L, increased by 49.32% compared to original strain, and bacitracin titer produced by the final strain DW2-CQD (DW2ΔcydB::qoxA::dck) was 954.25 U/mL, increased by 21.66%. The bacitracin titer produced per cell was 2.11 U/CFU, increased by 11.05%. Collectively, this study demonstrates that improving ATP content was an efficient strategy to improve bacitracin production, and a promising strain B. licheniformis DW2-CQD was attained for industrial production of bacitracin.

Enhanced production of bacitracin by knocking out of amino acid permease gene yhdG in Bacillus licheniformis DW2 / 生物工程学报

Bacitracin is a broad-spectrum polypeptide antibiotic, which is formed by 11 amino acids residues. Precursor amino acids supply might be the limit factor during bacitracin fermentation. First, our results demonstrated that increasing Ile and Leu supplies were regarded as the efficient strategies for the enhanced titer of bacitracin. Then, the amino acid permease YhdG, which was identified as the BCAA permease, was deleted and overexpressed in DW2, respectively. Our results showed that knocking out of permease YhdG could improve bacitracin production remarkablely. The bacitracin titer of the yhdG deficient strain DW2ΔyhdG reached 917.35 U/mL by flask fermentation, increased by 11% compared with that of DW2. In addition, the bacitracin titer was decreased by 25% in the YhdG overexpressed strain. Meanwhile, the intracellular concentrations of BCAA were higher than DW2 during the biosynthesis of bacitracin. The above results suggested that the permease YhdG might act as an exporter for branched chain amino acids in B. licheniformis DW2. Taken together, the increasing intracellular concentrations of branched chain amino acids by deleting amino acid permease YhdG could improve bacitracin titer. This study provided a new strategy for high-level production of bacitracin.

Effect of overexpressing Nisin A structural gene nisA on Nisin A production / 生物工程学报

Nisin is an antimicrobial peptide widely used in food industry. In this study, Nisin A production in Lactococcus lactis ATCC 11454 was improved by overexpression of Nisin A structural gene nisA through introducing a shuttle expression vector pMG36ek-nisA and an integrated vector pDG780-nisA into the host strain. The differences of growth profiles and Nisin A production level between the two obtained genetic engineering strains FMM1/FMM2 and the parent strain were investigated. Our results show that while the growth profile (the growth rate, biomass and pH) of FMM1 was similar to the parent strain, its Nisin A production increased 31%. In contrast, the biomass of FMM2 was notably lower than the parent strain, while its yield of Nisin A enhanced slightly. The transcription level of genes involved in Nisin A biosynthesis in both engineering strains was further detected by RT-PCR. We found that all the 11 Nisin A biosynthetic genes in FMM1 and FMM2 had a higher transcription level than those in the parent strain, and these genes exhibited more significant increasing degree of transcription level in FMM1 which hosted the autonomous replicating nisA gene. These data suggest that expression of nisA may act as a rate-limit factor in Nisin A biosynthesis. In conclusion, this work provides a new method to improve Nisin A production by increasing the transcription level of nisA, paving the way to further large-scale industrial production of Nisin A.